A pivot assembly bearing device used for a swing arm of a hard disk drive (HDD) is normally configured by inserting a pair of rolling bearings into a shaft. Grease is enclosed in these rolling bearings. Grease is lubricant made into semi-solid form by mixing base oil and thickener. As the grease for the pivot assembly bearing device, for example, urea-based grease containing hydrocarbon-based base oil such as polyalphaolefin or mineral oil and diurea compound as thickener can be used. Recently, in order to maintain superiority to a solid state drive (SSD), the HDD is getting larger in capacity, and higher densification is required. Therefore, a gap between a magnetic head and a magnetic disk is further reduced as compared with the prior art, and the adhesion of oil to a head part of the HDD is becoming a problem. It is considered that the adhesion of oil is caused by the base oil of the grease enclosed in the rolling bearing of the pivot assembly bearing device being oozed out to the outside through a metal surface. Accordingly, there is a demand for a pivot assembly bearing device capable of suppressing the oozing of the base oil of the grease to the outside. Many such inventions have been made.
For example, JP-A-H11-062972 discloses a rolling bearing for swinging motion as a rolling bearing for supporting an actuator of an HDD. In this rolling bearing, balls are disposed between an inner ring and an outer ring, a shield attached to the outer ring faces a sealing surface (outer peripheral surface) of the inner ring, lubricant is put into the bearing internal space defined by the outer ring, the inner ring and the shield, and oil-repellent agent is applied to the sealing surface and the surface (an inner diameter side end surface of the shield) of the shield facing the sealing surface. In this manner, even when grease is used as lubricant inside the bearing, the grease is repelled by the oil-repellent agent and does not ooze out of the bearing.
On the other hand, JP-A-H11-190353 discloses a rolling bearing device for a swing arm. In this rolling bearing device, lubricant is applied to a raceway surface of a rolling bearing, an inner ring and an outer ring of the rolling bearing are press-fitted respectively to an outer side surface of a shaft and an inner side surface of a housing, and oil-repellent agent is applied to an inner peripheral surface of an end portion of the housing. With this oil-repellent agent, lubricant oozing out from a gap between the inner ring and a seal of the rolling bearing and a small gap in a fixing portion of the seal and the outer ring is repelled by the oil-repellent agent, so that the leakage of the lubricant from the bearing device to the outside can be suppressed.
Here, in the pivot assembly bearing device as described above, the oil-repellent agent is applied to the inner ring and the shield (sealing plate) of the rolling bearing and the inner surface of the end portion of the housing incorporating the rolling bearing. However, it is difficult to apply the oil-repellent agent to the inner ring and the sealing plate of the extremely small ball bearing as used for the pivot assembly bearing device. That is, since the oil-repellent agent has low viscosity, it is likely to spread beyond the application region after applied and before it is dried to form the oil-repellent film. Therefore, it is difficult to form the oil-repellent film having an extremely small width on the inner diameter side end surface of the sealing plate and the outer peripheral surface of the inner ring of the extremely small ball bearing because the dimensions of the parts are small. Also, the oil-repellent film formed on the inner diameter side end surface of the sealing plate and the outer peripheral surface of the inner ring is easily peeled off. Therefore, even when the oil-repellent film is formed at the stage where the ball bearing has not assembled yet, there is a possibility that the oil-repellent film may be partially peeled off by rubbing against another member when assembling the ball bearing. Then, not only the failure of the oil-repellent film occurs, but also the oil-repellent film that is peeled off contaminates the inside of the bearing, which adversely affects the performance of the ball bearing. For these reasons, the pivot assembly bearing device in which the oil-repellent agent is applied to the rolling bearing has not been put to practical use.
On the other hand, it is also difficult to form the oil-repellent film by applying oil-repellent agent on the inner peripheral surface of the end portion of the housing to which the rolling bearing is fixed. That is, when the oil-repellent film is formed before the rolling bearing is assembled to the housing, the oil-repellent film and the outer peripheral surface of the rolling bearing rub against each other at the time of assembling the rolling bearing, and the oil-repellent film is peeled off. Further, after the rolling bearing is fixed to the housing, the low-viscosity oil-repellent agent may drip and enter the bearing or may adhere to the outer peripheral surface of the housing. The oil-repellent agent penetrated and dried in the bearing is a foreign matter and contaminates the rolling bearing. Also, the oil-repellent agent adhered and dried on the outer peripheral surface of the housing may cause poor adhesion when fixing the housing to the swing arm by an adhesive.